Power supply over-voltage protection

ABSTRACT

A voltage doubler (2) having a series arrangement of two smoothing capacitors (C1,C2), is able to use less expensive smoothing capacitors (C1,C2) having a far lower rated voltage which is only slightly above the maximum voltage occurring during normal operation. To this end, a first and a second over-voltage prevention circuit (30,31) are coupled across the smoothing capacitors (C1,C2) of the voltage doubler (2) to protect the smoothing capacitors (C1,C2) against over-voltage if one of the smoothing capacitors (C1,C2) is short-circuited. The first and the second over-voltage prevention circuit (30,31 ) may both be an over-voltage detector, or an under-voltage detector each being coupled across one of the smoothing capacitors (C1,C2). It is also possible to combine an over-voltage detector coupled across one of the smoothing capacitors (C1,C2) to an under-voltage detector coupled across the same smoothing capacitor (C1,C2). The over-voltage detector prevents an over-voltage across said one smoothing capacitor (C1,C2) if the other smoothing capacitor (C1,C2) is short-circuited. The under-voltage detector detects a short circuit of the one smoothing capacitor (C1,C2) and thereby prevents an over-voltage across the other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a power supply input circuit comprising adisconnecting circuit coupled to receive an AC-input voltage and adisconnecting signal for disconnecting the AC-input voltage from outputterminals of the disconnecting circuit if the disconnecting signal isactive, a series arrangement of a first and a second smoothing capacitorcoupled between output terminals of the power supply input circuit, avoltage doubler circuit coupled to said output terminals for supplying arectified output voltage across the series arrangement of the first andthe second smoothing capacitor in a first situation where no voltagedoubling is required, and for supplying one polarity of the AC-inputvoltage across said first smoothing capacitor and the other polarity ofthe AC-input voltage across said second smoothing capacitor, to providea doubled rectified output voltage across the series arrangement of thefirst and the second smoothing capacitor in a second situation where avoltage doubling is required, and an over-voltage detection circuithaving an output for supplying the disconnecting signal.

The invention also relates to an over-voltage protection method ofprotecting smoothing capacitors in a voltage doubler, the voltagedoubler receiving an AC-input voltage and comprising a seriesarrangement of two smoothing capacitors coupled between output terminalsof the voltage doubler, while in a first situation where no voltagedoubling is required, the voltage doubler supplies a rectified outputvoltage across the series arrangement of the two smoothing capacitors,and in a second situation where a voltage doubling is required, thevoltage doubler supplies one polarity of the AC-input voltage across oneof the smoothing capacitors and supplies the other polarity of theAC-input voltage across the other smoothing capacitor to provide adoubled rectified voltage across the series arrangement of the twosmoothing capacitors, the method comprising the steps of: detecting anover-voltage, generating an active disconnecting signal if anover-voltage is detected, and disconnecting the AC-input voltage fromthe voltage doubler in response to the active disconnecting signal.

A power supply input circuit comprising a voltage doubler and anover-voltage circuit may be used, for example in apparatuses which haveto operate on a large range of AC-input voltages. Low AC-input voltagesbeing about a magnitude of two lower than high AC-input voltages mayoccur in this large range of AC-input voltages. Doubling the lowAC-input voltages with the voltage doubler causes a smaller inputvoltage range of a stabilizing power supply succeeding the power supplyinput circuit.

2. Description of the Related Art

Such a power supply input circuit is known from JP-A-1-185,133. Theknown power supply input circuit comprises rectifier diodes arranged ina full-wave rectifier bridge, this bridge having two output terminalswhich are connected to a series arrangement of two smoothing capacitors.The voltage across the series arrangement of the two smoothingcapacitors is the output voltage of the power supply input circuit. Thefull-wave rectifier bridge further comprises two input terminals, one ofwhich is connected to one terminal of an AC-input voltage source via afuse. At high AC-input voltages (for example, 220 V) the other inputterminal of the full-wave rectifier bridge is connected to the otherterminal of the AC-input voltage source via a switch being in a firstposition, to supply a full-wave rectifier AC-input voltage to the outputterminals of the power supply input circuit. At low AC-input voltages(for example, 110 V) the switch is in a second position to connect theother terminal of the AC-input voltage source to a connection node ofthe two smoothing capacitors to supply a doubled AC-input voltage to theoutput terminals of the power supply input circuit (the other terminalof the full-wave rectifier bridge is not connected, and only tworectifier diodes are operational). An avalanche diode is connected inparallel to each smoothing capacitor. If the switch is erroneously inthe second position at high AC-input voltages, an over-voltage on thesmoothing capacitors is prevented because the avalanche diodes willstart conducting, thereby causing a large short-circuit current thatblows the fuse. A drawback of the known power supply input circuit isthat the rectifier diodes have to cope with the large short circuitcurrent. An over-voltage is herein understood to be a voltage higherthan the rated voltage of the smoothing capacitors. The rated voltage isthe maximum operating voltage the smoothing capacitors are designed for.

Such a power supply input circuit is also known from a Sanken ElectricCompany publication SSE-17284E which describes an application circuit ofa voltage doubler Integrated Circuit STR81000A. This known circuit againcomprises a fuse, rectifier diodes arranged in a full wave rectifierbridge, a semiconductor switch, and two smoothing capacitors, arrangedand operating in the same way as described above. The integrated circuitcomprises the switch and electronics to determine the right position ofthe switch depending on the level of the AC-input voltage. Over-voltageon the smoothing capacitors as a consequence of a wrong position of theswitch at a high AC-input voltage is now prevented by causing athyristor to conduct if a too high voltage across the series arrangementof the smoothing capacitors is detected. As the thyristor precedes therectifier bridge, a large short circuit current for blowing a fuse willnot flow through the rectifier diodes. Thus, this known power supplyinput circuit solves the problem of the first-mentioned known powersupply input circuit, but has the drawback that a short circuit of onlyone of the smoothing capacitors is not detected. In that case a voltage(for example, 390 V) which is far higher than the rated voltage (forexample, 250 V) of the smoothing capacitors will occur across thesmoothing capacitor which is not short-circuited. Such a high voltagecan cause a breakdown of this smoothing capacitor in a dangerous way.

SUMMARY OF THE INVENTION

Starting from the last-mentioned prior art it is, inter alia, an objectof the invention to provide a power supply input circuit comprising anover-voltage detection circuit which also prevents a dangerous situationif an over-voltage occurs on only one of the smoothing capacitors,without causing a large current through rectifier means for rectifyingan AC-input voltage.

The power supply input circuit in accordance with the invention ischaracterized in that the over-voltage detection circuit comprises afirst over-voltage prevention circuit to detect an over-voltage acrosssaid first smoothing capacitor, and a second over-voltage preventioncircuit to detect an over-voltage across said second smoothingcapacitor, the over-voltage detection circuit activating thedisconnecting signal if an overvoltage is detected across at least oneof said smoothing capacitors.

An over-voltage protection method of protecting smoothing capacitors ina voltage doubler is characterized in that the detection of theover-voltage situation comprises the steps of: detecting an over-voltageacross said first smoothing capacitor, detecting an over-voltage acrosssaid second smoothing capacitor, and furnishing said disconnectingsignal if an over-voltage across at least one of said first and saidsecond smoothing capacitors is detected.

A voltage-doubler receives an AC-input voltage, and comprises a firstand a second smoothing capacitor arranged in series between outputterminals of the power supply input circuit. The voltage doubler can bebuilt up as described in the recited prior art. The rectifier diodes canbe replaced by other rectifier elements (for example thyristors), andneed not be arranged as a full-wave rectifier bridge: also at highAC-input voltages a half-wave rectifier arrangement can be used,comprising, for example, one rectifier element. Independent of theconstruction of the voltage doubler, the output terminals of the powersupply input circuit supply a rectified AC input voltage across theseries arrangement of the first and the second smoothing capacitor ifthe AC-input voltage is above a certain input level (for example,AC-input voltages in the range from 160 V to 276 V), or the sum (sosubstantially a doubling of the AC-input voltage) of a rectifiedpositive polarity of the AC-input voltage across one of the smoothingcapacitors added to the rectified negative polarity of the AC-inputvoltage across the other smoothing capacitor if the AC-input voltage isbelow a certain level (for example, AC-input voltages in the range from85 V to 140 V). In accordance with the present invention, theover-voltage detection circuit comprises a first and a secondover-voltage prevention circuit to prevent an over-voltage across bothsmoothing capacitors. The over-voltage detection circuit activates thedisconnecting signal if the first or the second over-voltage preventioncircuit detects an over-voltage across a related smoothing capacitor.

An over-voltage prevention circuit may be an over-voltage detector whichis coupled across the smoothing capacitor, across which the over-voltagehas to be detected to supply an over-voltage signal for activating thedisconnecting signal if an over-voltage level is exeeded. Theover-voltage level has a value above the maximum voltage occurring onthe smoothing capacitor during normal operation and below the ratedvoltage of this smoothing capacitor. An over-voltage occurs on one ofthe smoothing capacitors if the other smoothing capacitor has been ashort-circuited, for example, due to ageing or overload.

An over-voltage prevention circuit may also be an under-voltage detectorcoupled across the smoothing capacitor across which the under-voltagehas to be detected to supply an under-voltage signal for activating thedisconnecting signal if the voltage across this smoothing capacitor isbelow a certain low level. The low level is chosen below the minimalvoltage across a smoothing capacitor in normal operation and above ashort-circuit voltage occurring across a short-circuited smoothingcapacitor. In this way, an over-voltage across the smoothing capacitorwhich is not short-circuited is prevented by detecting a short circuitof the other smoothing capacitor.

To prevent an over-voltage across each of the smoothing capacitors,three configurations are possible: an over-voltage detector is coupledacross each of the smoothing capacitors an under-voltage detector iscoupled across each of the smoothing capacitors or an over-voltagedetector is coupled across one of the smoothing capacitors and anunder-voltage detector is coupled across the same one of the smoothingcapacitors.

The voltage doubler will be disconnected from the AC-input voltagesource by a disconnecting circuit if the disconnecting signal isactivated.

An embodiment of the power supply input circuit in accordance with theinvention has the advantage that an over-voltage across each of thesmoothing capacitors is detected directly by over-voltage detectorsbeing coupled across each of the smoothing capacitors.

Another embodiment of the power supply input circuit in accordance withthe invention has the advantage that the under-voltage detectors have asimple and cheap construction.

A further embodiment of the power supply input circuit in accordancewith the invention has the advantage that both the over-voltage detectorand the under-voltage detector are coupled across the same smoothingcapacitor. In this way, the disconnecting signal can be activated by asimple combination of the over-voltage signal and the under-voltagesignal to avoide bridging the large voltage difference between relatedconnections of the first and the second smoothing capacitor.

A further embodiment of the power supply input circuit in accordancewith the invention is characterized by coupling one of the connectionsof the first capacitor to ground, the combination of the over-voltagesignal and the under-voltage signal yields a disconnecting signal withreference to ground. This disconnecting signal can then be easily usedto activate the disconnecting circuit. This is especially the case ifthe disconnecting circuit comprises a fuse arranged between the AC-inputvoltage source and an input terminal of the voltage doubler and athyristor coupled between this input terminal of the voltage doubler andground, the cathode of the thyristor being coupled to ground. An activedisconnecting signal with respect to ground can easily trigger thethyristor to become conductive so as to blow the fuse. The largeshort-circuit current which flows through the fuse and the thyristordoes not flow through the elements of the voltage doubler circuit.

A further embodiment of the power supply input circuit in accordancewith the invention is characterized in that the over-voltage detectorcomprises a parallel arrangement of a zener diode and a third resistor,the parallel arrangement being coupled across one of the smoothingcapacitors, in that a base of a bipolar transistor is coupled to ajunction of the two resistors, and an emitter of the bipolar transistoris coupled to a junction of the third resistor, and in that the zenerdiode is oriented in such a way that an emitter current flowing in theemitter of the bipolar transistor flows from cathode to anode of thezener diode, and in that a collector of the bipolar transistor isconnected to the output of the over-voltage detection circuit to supplythe disconnecting signal. In this embodiment a compensation of thetemperature-coefficient of the bipolar transistor and that of thezener-diode is achieved, thereby gaining a stabler over-voltage triggerlevel.

In a further embodiment of the power supply input circuit in accordancewith the invention, the over-voltage detection circuit is adapted sothat no over-voltage detection will occur if the duration of theover-voltage is too short to cause a failure of one of the smoothingcapacitors.

A second aspect of the invention provides a picture display apparatuscomprising a power supply input circuit as described above, a main powersupply coupled to the output terminals of the power supply input circuitto supply at least one stabilized output voltage, and a horizontaladdressing circuit receiving said stabilized output voltage to supply ahorizontal addressing signal for determining a horizontal position on adisplay device and a vertical addressing signal for determining avertical position on the display device. The limiting of the maximumvoltage across the smoothing capacitors by the over-voltage preventioncircuits also limits the maximum voltage at the output terminals of thepower supply input circuit and thereby reduces the over-voltagevoverload on a succeeding power supply. This power supply may be morereliable and cheaper. Furthermore the input voltage range of thesucceeding power supply is much smaller, which enables a betterstabilisation of output voltages or a simpler design of the powersupply.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of the invention will now be describedin more detail by way of example with reference to the accompanyingdrawings, in which:

FIG. 1 shows a basic block diagram of a picture display apparatuscomprising a power supply input circuit according to the invention;

FIG. 2 shows an embodiment of an over-voltage detection circuitaccording to the invention;

FIG. 3 shows another embodiment of an over-voltage detection circuitaccording to the invention; and

FIG. 4 shows yet another embodiment of an over-voltage detection circuitaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a basic block diagram of a picture display apparatuscomprising a power supply input circuit 1,2,3 according to theinvention. A disconnecting circuit 1 is coupled to receive an AC-inputvoltage Vac and a disconnecting signal Vd for disconnecting the AC-inputvoltage from output terminals T1,T2 of the disconnecting circuit 1 ifthe disconnecting signal Vd is active. A voltage-doubler 2 comprises avoltage doubler circuit 20 coupled to the output terminals T1,T2 of thedisconnecting circuit 1. The voltage doubler circuit 20 has a firstdoubler output terminal T3 which is a first output terminal of the powersupply input circuit, a second doubler output terminal T4 which is asecond output terminal of the power supply input circuit, and a thirddoubler output terminal T5. The voltage doubler 2 further comprises afirst and a second smoothing capacitor C1,C2 arranged in series betweenthe first and the second doubler output terminals T3,T4. The firstsmoothing capacitor C1 is coupled between the doubler output terminalsT4,T5. The second smoothing capacitor C2 is coupled between the doubleroutput terminals T3,T5. An overvoltage detection circuit 3 is coupled totwo of the doubler output terminals T3,T4,T5, if an over-voltagedetector and an under-voltage detector is used, both being coupledacross the same one of the smoothing capacitors C1,C2. The over-voltagedetection circuit 3 is coupled to all three doubler output terminalsT3,T4,T5, if an over-voltage detector or an under-voltage detector iscoupled across each smoothing capacitor C1 ,C2. The over-voltagedetection circuit 3 supplies the disconnecting signal Vd to thedisconnecting circuit 1. The disconnecting signal Vd is active if avoltage across the first or the second smoothing capacitor C1,C2 ishigher than a maximum permissible value. This maximum permissible valueis chosen above the maximum voltage occurring on each smoothingcapacitor during normal operation of the power supply input circuit andbelow the rated voltage of each smoothing capacitor.

The disconnecting circuit 1 comprises, for example, a fuse arranged inseries with an AC voltage source Vac and an element for generating alarge current through the fuse (for example, a thyristor coupled betweenthe fuse and ground) to disconnect the voltage doubler 2 from theAC-voltage source Vac.

The voltage doubler circuit 20 comprises four rectifying elementsD1,D2,D3,D4 arranged in a bridge configuration and a switch S1 havingtwo positions. The way in which this voltage doubler circuit operates isnot further elucidated, as it is known from the prior art.

A main power supply 4 is coupled to the output terminals T3,T4 of thepower supply input circuit 1,2,3 to supply at least one sufficientlystabilized supply-voltage Vs to an addressing stage 5. The addressingstage 5 supplies horizontal addressing information Hadr and verticaladdressing information Vadr to a picture device 6 for determining ahorizontal and a vertical position on a picture display screen of thepicture device 6.

If the voltage doubler 2 is in the doubling mode and one of thesmoothing capacitors C1,C2 is short-circuited, a very largeshort-circuit current will flow through the short-circuited smoothingcapacitor C1,C2, the conducting rectifier diode D1,D2,D3,D4 and thedisconnecting circuit 1. This large short-circuit current can also beused to activate the disconnection of the voltage doubler 2 from theAC-input voltage source Vac (for example by blowing a fuse arrangedbetween the AC-input voltage source Vac and one of the input terminalsT1 ,T2 of the voltage doubler 2). Thus, no over-voltage detection acrosseach of the smoothing capacitors C1, C2 is necessary.

If the voltage doubler 2 is not in the doubling mode, the two smoothingcapacitors C1,C2 are arranged in series, and no large short-circuitcurrent will flow through the series arrangement of the two smoothingcapacitors C1,C2 if one of the smoothing capacitors C1,C2 isshort-circuited. Consequently, a fuse arranged as mentioned above willnot disconnect. In this case, the rectified voltage occurring in normaloperation across the series arrangement of the smoothing capacitorsC1,C2 now occurs only across the smoothing capacitor C1 or C2 which isnot short-circuited. This would be no problem if expensive smoothingcapacitors C1,C2 having a high rated voltage specification were selected(400 V if the maximum AC input voltage Vac is 276 V). By selecting arated voltage of 250 V for each smoothing capacitor C1,C2, cheapersmoothing capacitors can be used, but then a protection against anover-voltage (400 V if one of the smoothing capacitors isshort-circuited) across the smoothing capacitors C1,C2 is necessary. Asin normal operation the voltage across each smoothing capacitor is about200 V, the area between 200 V and 250 V can be used to detect anover-voltage situation.

FIGS. 2 and 3 show embodiments of an over-voltage detection circuit 3according to the invention. In these embodiments, a first over-voltageprevention circuit 30 is coupled across the first smoothing capacitorC1, and a second over-voltage prevention circuit 31 is coupled acrossthe second smoothing capacitor C2. Both the first and the secondovervoltage prevention circuits 30,31 are an over-voltage detector.

In FIG. 2, the first over-voltage prevention circuit 30 comprises asecond zener diode Z2, a second diode D6, a bipolar transistor Tr1, anda first, a second and a third resistor R1,R2,R3. The second detector 31comprises a first zener diode Z1 having a cathode which is coupled tothe first doubler output terminal T3, and an anode which is coupled to ajunction of a cathode of the second diode D6 and a cathode of the secondzener diode Z2. An anode of the second diode D6 is coupled to the thirddoubler output terminal T5. An anode of the second zener diode Z2 iscoupled to an emitter of the bipolar pnp transistor Tr1 and to the thirdresistor R3. The other side of the third resistor R3 is coupled to thesecond doubler output terminal T4. The first resistor R1 is coupledbetween the anode of the second diode D6 and a base of the bipolar pnptransistor Tr1. The second resistor R2 is coupled between the base ofthe bipolar pnp transistor Tr1 and the second doubler output terminalT4. A collector of the bipolar transistor Tr1 is coupled to supply anintermediate disconnecting signal Vd'.

The intermediate disconnecting signal Vd' becomes active if the bipolarpnp transistor Tr1 is turned on, which is the case if the voltage acrossthe first smoothing capacitor C1 exceeds a certain level, or when thefirst zener diode Z1 becomes conducting, which is the case if thevoltage across the second smoothing capacitor C2 exceeds a certainlevel. This level is chosen between the rated voltage of the smoothingcapacitors C1,C2 (250 V) and the maximum voltage (200 V) across one ofthe smoothing capacitors C1,C2 in normal non-doubling operation of thevoltage doubler 2. Consequently, the zener voltage of the first zenerdiode Z1 (which may be a series arrangement of zener diodes) could beselected to be 225 V. The values of the components of the firstover-voltage prevention circuit 30 could be R1=10 kohms, R2=135 kohms,zener voltage VZ2=15 V, so that the certain level isVtr=(2*Vbe+VZ2)*(R1+R2)/R1=237,8 V, wherein 2*Vbe=1.4 V is constitutedby the base-emitter voltage of the first transistor Tr1 and the voltageacross the second diode D6.

An over-voltage across the smoothing capacitor C1,C2 is allowed to occurduring a short period without causing the smoothing capacitor C1,C2 tofall. Such an overvoltage during a short period can be caused, forexample, by a sudden increase of a mains voltage, as is the case after amains dip or during a mains spike. It is an advantage if theover-voltage detection does not become active in these situations. Asimple solution is obtained by adding a smoothing circuit R5,C4 forsupplying an active disconnecting signal Vd if the intermediatedisconnecting signal Vd' is active during a sufficiently long period. Toachieve this, a fourth resistor R4 is connected between the collector ofthe bipolar pnp transistor Tr1 and a junction of a cathode of a secondzener diode Z3, a collector of a bipolar npn transistor Tr2 and a fifthresistor R5. A base of the bipolar npn transistor Tr2 is coupled to ajunction of a capacitor C3 and a unconnected end of the fifth resistor1t5. An emitter of the bipolar npn transistor Tr2 supplies thedisconnecting signal Vd and is coupled to a sixth resistor R6. An anodeof the second zener diode Z3, a unconnected end of the capacitor C3, anda unconnected end of the sixth resistor R6 are coupled to ground. Thefourth resistor R4 and the second zener diode Z3 limit the voltage onthe collector of the bipolar npn transistor Tr2. The fifth resistor R5and the third capacitor C3 cause the disconnecting signal Vd to becomeactive only if the intermediate disconnecting signal Vd' is activeduring a period which is long enough to charge the capacitor C3.

In FIG. 3, the first over-voltage prevention circuit 30 is essentiallythe same as that described in FIG. 2, but the second diode D6 isreplaced by a short circuit, a capacitor C3 is connected between thebase of the bipolar pnp transistor and the cathode of the zener diodeZ2, and a further diode D7 is added and has an anode coupled to thecollector of the pnp transistor Tr1 and a cathode coupled to an outputterminal of the first over-voltage prevention circuit 30. The secondover-voltage prevention circuit 31 is the same circuit as the firstover-voltage prevention circuit 30 of FIG. 3. Elements of the secondover-voltage prevention circuit 31 are indicated by the same referencenumerals as corresponding elements of the first over-voltage preventioncircuit 30, but followed by an apostrophe. An output terminal of thesecond over-voltage prevention circuit 31 is coupled to the outputterminal of the first over-voltage prevention circuit 30 to supply adisconnecting signal Vd. The disconnecting signal Vd becomes active if acertain level across one of the smoothing capacitors is crossed, asexplained in the description of FIG. 2. An advantage of this embodimentis a more accurate over-voltage detection by having a resistive dividerR1',R2',R1,R2 across the series arrangement of the smoothing capacitorsC1,C2. By using the resistive divider, the voltage division across thesmoothing capacitors C1,C2 is no longer dependent on a large spread ofthe values of the smoothing capacitors C1,C2. As an over-voltage acrossa smoothing capacitor C1,C2 is allowed to occur during a short period,the capacitors C3,C3' are added. A sudden increase of a voltage across asmoothing capacitor C1,C2 will also occur across the capacitor C3,C3',thereby preventing the bipolar pnp transistor from starting being turnedon immediately.

FIG. 4 shows another embodiment of an over-voltage detection circuit 3according to the invention. The first over-voltage prevention circuit 30is again an over-voltage detector coupled across the first smoothingcapacitor C1 which may contain the same circuit as the firstover-voltage prevention circuit 30 as described in the embodiment ofFIG. 3. In this case the first over-voltage prevention circuit 30comprises a zener diode Z1 having a cathode coupled to the third doubleroutput terminal T5 and an anode coupled to a junction of a resistor R1and an output terminal to supply the disconnecting signal Vd. The otherend of the resistor R1 is coupled to ground. The disconnecting signalbecomes active if the voltage across the first smoothing capacitor C1crosses a zener voltage of the zener diode Z1. The first over-voltageprevention circuit 30 detects an over-voltage across the first smoothingcapacitor C1 caused by a short-circuited second smoothing capacitor C2.The second over-voltage prevention circuit 31 is an under-voltagedetector coupled across the first smoothing capacitor C1 to detect ashort circuit of the first smoothing capacitor C1 for preventing anover-voltage on the second smoothing capacitor C2. The secondover-voltage prevention circuit 31 comprises a first resistor R2 coupledbetween the third doubler output terminal T5 and a base of a bipolar npntransistor Tr1. A collector of the bipolar npn transistor Tr1 is coupledto a second resistor R3 and to a third capacitor C3. An emitter of thebipolar npn transistor and another side of the third capacitor C3 arecoupled to the second doubler output terminal T4 which is coupled toground. Another end of the second resistor R3 is coupled to a suitablevoltage supply source Vb. A diode D5 has an anode coupled to the otherside of the second resistor R3 and a cathode coupled to the outputterminal of the first over-voltage prevention circuit 30 to supply thedisconnecting signal Vd. At the instant when the first smoothingcapacitor C1 is a short-circuited the bipolar npn transistor Tr1 will beturned off, thereby activating the disconnecting signal Vd so as toprevent an over-voltage from occurring across the second smoothingcapacitor C2.

It will be evident that variants of the embodiments described arepossible within the scope of the invention. For example, pnp transistorTr1 ,Tr1' used in an over-voltage detector could as well be a npntransistor having an emitter coupled to a doubler output terminal T4,T5via a zener diode Z2,Z2'. Also npn transistor Tr1 used in anunder-voltage detector could as well be a pnp bipolar transistor havingan emitter coupled to one of the doubler output terminals T5 and acollector coupled to a suitable voltage via a resistor. In these casesthe disconnecting signal is active if it has a low level. The transistorTr1 ,Tr1' may also be a FET. The zener diodes Z2,Z2' used in anover-voltage detector may be replaced by a resistor.

In brief the invention is summarized below. Prior art voltage doublers 2comprise a series arrangement of two smoothing capacitors C1,C2 having arated voltage which is chosen at least twice higher than the maximumoperating voltage occurring across each smoothing capacitor C1,C2 innormal operation. If one of the smoothing capacitors C1,C2 isshort-circuited, the other one of the smoothing capacitors C1,C2 cancope with the voltage across it which is about twice as high. Theinvention is able to use less expensive smoothing capacitors C1,C2having a far lower rated voltage which is only slightly above themaximum voltage occurring during normal operation. To this end, a firstand a second over-voltage prevention circuit 30,31 are coupled acrossthe smoothing capacitors C1,C2 of the voltage doubler 2 to protect thesmoothing capacitors C1,C2 against over-voltage if one of the smoothingcapacitors C1,C2 is short-circuited. The first and the secondover-voltage prevention circuit 30,31 may both be an over-voltagedetector or an under-voltage detector each being coupled across one ofthe smoothing capacitors C1,C2. It is also possible to combine anover-voltage detector coupled across one of the smoothing capacitorsC1,C2 with an under-voltage detector coupled across the same smoothingcapacitor C1,C2. The overvoltage detector prevents an over-voltageacross said one smoothing capacitor C1,C2 if the other smoothingcapacitor C1,C2 is a short circuit. The under-voltage detector detects ashort circuit of the one smoothing capacitor C1,C2 and thereby preventsan over-voltage across the other.

We claim:
 1. A power supply input circuit (1,2,3) comprisingadisconnecting circuit (1) coupled to receive an AC-input voltage (Vac)and a disconnecting signal (Vd) for disconnecting the AC-input voltage(Vac) from output terminals (T1,T2) of the disconnecting circuit (1) ifthe disconnecting signal (Vd) is active, a series arrangement of a firstand a second smoothing capacitor (C1,C2) coupled between outputterminals (T3,T4) of the power supply input circuit (1,2,3), a voltagedoubler circuit (20) coupled to said output terminals (T1,T2) forsupplying a rectified output voltage across the series arrangement ofthe first and the second smoothing capacitor (C1,C2) in a firstsituation where no voltage doubling is required, and for supplying onepolarity of the AC-input voltage (Vac) across said first smoothingcapacitor (C1) and the other polarity of the AC-input voltage (Vac)across said second smoothing capacitor (C2), to provide a doubledrectified output voltage across the series arrangement of the first andthe second smoothing capacitor (C1,C2) in a second situation where avoltage doubling is required, and an over-voltage detection circuit (3)having an output for supplying the disconnecting signal(Vd),characterized in that the over-voltage detection circuit (3)comprises a first over-voltage prevention circuit (30) to detect anover-voltage across said first smoothing capacitor (C1), and a secondover-voltage prevention circuit (31) to detect an over-voltage acrosssaid second smoothing capacitor (C2), the over-voltage detection circuit(3) activating the disconnecting signal (Vd) if an over-voltage isdetected across at least one of said smoothing capacitors (C1,C2).
 2. Apower supply input circuit as claimed in claim 1, characterized in thatthe first over-voltage prevention circuit (30) includes a firstover-voltage detector which is coupled across the first smoothingcapacitor (C1) to activate the disconnecting signal (Vd) if a voltageacross the first smoothing capacitor (C1) is above a certain level, andthe second over-voltage prevention circuit (31) includes a secondover-voltage detector which is coupled across the second smoothingcapacitor (C2) to activate the disconnecting signal (Vd) if a voltageacross the second smoothing capacitor (C2) is above a certain level. 3.A power supply input circuit as claimed in claim 1, characterized inthat the first over-voltage prevention circuit (30) includes a firstunder-voltage detector which is coupled across the first smoothingcapacitor (C1) to activate the disconnecting signal (Vd) if a voltageacross the first smoothing capacitor (C1) is below a certain value, andthe second over-voltage prevention circuit (31) includes a secondunder-voltage detector which is coupled across the second smoothingcapacitor (C2) to activate the disconnecting signal (Vd) if a voltageacross the second smoothing capacitor (C2) is below a certain value. 4.A power supply input circuit as claimed in claim 1, characterized inthat the first over-voltage prevention circuit (30) includes anover-voltage detector which is coupled across one of the smoothingcapacitors (C1,C2) to activate the disconnecting signal (Vd) if acertain voltage across said one of the smoothing capacitors (C1,C2) isabove a certain level, and the second over-voltage prevention circuit(31) includes an under-voltage detector which is coupled across said oneof the smoothing capacitors (C1,C2) to activate the disconnecting signal(Vd) if the voltage across said one of the smoothing capacitors (C1,C2)is below a certain value.
 5. A power supply input circuit as claimed inclaim 4, characterized in that one terminal of said one (C1) of thesmoothing capacitors (C1,C2) is coupled to ground.
 6. A power supplyinput circuit as claimed in claim 2, characterized in that theover-voltage detector comprises a parallel arrangement of twoseries-arranged resistors (R1,R2;R1',R2') and a series arrangement of azener diode (Z2;Z2') and a third resistor (R3;R3'), the parallelarrangement being coupled across one of the smoothing capacitors(C1,C2), in that a base of a bipolar transistor (Tr1;Tr1') is coupled toa junction of the two resistors (R1,R2;R1',R2'), and an emitter of thebipolar transistor (Tr1;Tr1') is coupled to a junction of the thirdresistor (R3;R3'), and in that the zener diode (Z2;Z2') is oriented insuch a way that an emitter current flowing in the emitter of the bipolartransistor (Tr1;Tr1') flows from cathode to anode of the zener diode(Z2;Z2'), and in that a collector of the bipolar transistor (Tr1;Tr1')is connected to the output of the over-voltage detection circuit (3) tosupply the disconnecting signal (Vd).
 7. A power supply input circuit asclaimed in claim 2, characterized in that the over-voltage detectioncircuit (3) is adapted not to supply an active disconnecting signal (Vd)to the disconnecting circuit (1) if the over-voltage detected by theover-voltage detector occurs during a period which is too short to causea failure of one of the smoothing capacitors (C1,C2).
 8. A power supplyinput circuit as claimed in claim 7, characterized in that a smoothingcircuit (R5,C4) is coupled between the output of the over-voltagedetection circuit (3) and the disconnecting circuit (1).
 9. A powersupply input circuit as claimed in claim 7, characterized in that theover-voltage detector comprises a parallel arrangement of twoseries-arranged resistors (R1,R2;R1',R2') and a series arrangement of azener diode (Z2;Z2') and a third resistor (R3;R3'), the parallelarrangement being coupled across one of the smoothing capacitors(C1;C2), in that a base of a bipolar transistor (Tr1;Tr1') is coupled toa junction of the two resistors (R1,R2;R1',R2'), and an emitter of thebipolar transistor (Tr1;Tr1') is coupled to a junction of the thirdresistor (R3;R3'), and in that the zener diode (Z2;Z2') is oriented insuch a way that an emitter current flowing in the emitter of the bipolartransistor (Tr1;Tr1') flows from cathode to anode of the zener diode(Z2;Z2'), in that a collector of the bipolar transistor (Tr1;Tr1')supplies the disconnecting signal (Vd), and in that a capacitor (C3;C3')is coupled between the base of the bipolar transistor (Tr1;Tr1') and aconnection point of the zener diode (Z2;Z2') which is not coupled to theemitter of the bipolar transistor (TR1;TR1').
 10. A picture displayapparatus comprising:a power supply input circuit (1,2,3) as claimed inclaim 1, a main power supply (4) coupled to the output terminals (T3,T4)of the power supply input circuit (1,2,3) to supply at least onestabilized output voltage (Vs), and a horizontal addressing circuit (5)receiving said stabilized output voltage (Vs) to supply a horizontaladdressing signal (Hadr) for determining a horizontal position on adisplay device 6 and a vertical addressing signal (Vadr) for determininga vertical position on the display device (6).
 11. An over-voltageprotection method of protecting smoothing capacitors (C1,C2) in avoltage doubler (2), the voltage doubler (2) receiving an AC-inputvoltage (Vac) and comprising a series arrangement of two smoothingcapacitors (C1,C2) coupled between output terminals (T3,T5,T4) of thevoltage doubler (2), while in a first situation where no voltagedoubling is required, the voltage doubler (2) supplies a rectifiedoutput voltage across the series arrangement of the two smoothingcapacitors (C1,C2), and while in a second situation where a voltagedoubling is required, the voltage doubler (2) supplies one polarity ofthe AC-input voltage across one of the smoothing capacitors (C1,C2) andsupplies the other polarity of the AC-input voltage across the othersmoothing capacitor (C1,C2) to provide a doubled rectified voltageacross the series arrangement of the two smoothing capacitors(C1,C2),the method comprising the steps of: detecting (3) anover-voltage, generating an active disconnecting signal (Vd) if anover-voltage is detected, and disconnecting (1) the AC-input voltage(Vac) from the voltage doubler (2) in response to the activedisconnecting signal (Vd),characterized in that the detection (3) of theover-voltage situation comprises the steps of: detecting (30) anover-voltage across said first smoothing capacitor (C1), detecting (31)an over-voltage across said second smoothing capacitor (C2), andfurnishing said disconnecting signal (Vd) if an over-voltage across atleast one of said first and said second smoothing capacitors (C1,C2) isdetected.